This invention relates generally to analog low pass filters, and particularly to a filter that reduces switching events when the filter is switched in and out of a phase locked loop.
Conventional control systems such as phase locked loops used, for example, for frequency synthesis, often require a low pass filter in the forward path of the loop to suppress undesired circuit noise before the phased lock loop signal reaches the voltage controlled element. If the system has a high bandwidth, high-speed acquisition mode, the filter must be switched out of the signal path to avoid instability during signal acquisition. In a subsequent steady state narrow band mode, the filter must be switched back into the signal path after signal acquisition.
However, spurious transients are introduced when the filter is switched back into the signal path. The resulting partial reacquisition by the filter and the system of the signal thereby reduces the overall benefit of the acquisition mode. The spurious transients become even more problematic when they vary, as it is difficult to compensate for the varying transients. Further, the switching circuits and other elements of the filter often add unwanted active circuit random noise in the narrow bandwidth mode, thereby reducing the originally intended noise suppression benefits of the acquisition mode.
Therefore, it is an object of the present invention to provide a low pass filter that can be switched in and out of a control system in a manner that minimizes the effects of switching transients.
It is a further object of the present invention to provide a low pass filter for switching in and out of a control system in a manner that minimizes the effects of switching transients by producing a known consistent transient that allows for compensation.
It is a further object of the present invention to provide a low pass filter for switching in and out of a control system in a manner that minimizes unwanted active circuit random noise in the narrow band mode of operation.
In view of the above, the present invention provides a switched low pass filter that minimizes transients generated during filter switching events and that does not add active circuit random noise. The switched low pass filter includes a filter input terminal for receiving an input base band signal, an RC circuit for passing only a filtered portion of the input base band signal depending on a wide, mid or narrow band mode of filter operation, and an output terminal to connect to a following controlled element.
The switched low pass filter also includes a transient reduction circuit in switchable communication with the RC circuit for minimizing transients and switching events caused by transitioning to the mid and narrow band modes of filter operation, and a filter output terminal for outputting the input base band signal in the wide band mode of operation and the filtered portion of the base band signal subsequent to the transient reduction circuit minimizing the transients and the switching events caused by transitioning to the mid and narrow band modes of filter operation.